The present invention relates to a method for measuring critical dimensions on photomasks, and more specifically to a method for measuring critical dimensions on photomasks using an electrical test structure.
Photomasks are used in the semiconductor industry to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. The process for transferring an image from a photomask to a silicon substrate or wafer is commonly referred to as lithography or microlithography.
A typical binary photomask is comprised of a transparent quartz substrate and chrome (Cr) opaque material that includes an integral layer of chrome oxide (CrO) anti-reflective (AR) material. The pattern of the Cr opaque material and CrO AR material on the quartz substrate is a scaled negative of the image desired to be formed on the semiconductor wafer.
To create an image on a semiconductor wafer, a photomask is interposed between the semiconductor wafer, which includes a layer of photosensitive material, and an optical system. Energy generated by an energy source, commonly referred to as a Stepper, is inhibited from passing through the areas of the photomask in which the Cr opaque material and CrO AR is present. Energy from the Stepper passes through the transparent portions of the quartz substrate not covered by the Cr opaque material and the CrO AR material. The optical system projects a scaled image of the pattern of the opaque material onto the semiconductor wafer and causes a reaction in the photosensitive material on the semiconductor wafer. The solubility of the photosensistive material is changed in areas exposed to the energy. In the case of a positive photolithographic process, the exposed photosensistive material becomes soluble and can be removed. In the case of a negative photolithographic process, the exposed photosensistive material becomes insoluble and unexposed soluble photosensistive material is removed.
After the soluble photosensistive material is removed, the image or pattern formed in the insoluble photosensistive material is transferred to the substrate by a process well known in the art which is commonly referred to as etching. Once the pattern is etched onto the substrate material, the remaining resist is removed resulting in a finished product.
As described above, the image or pattern of the Cr opaque material and CrO AR material on the photomask reflects the desired image to be formed in the semiconductor wafer. It is therefore necessary that the image or pattern of the Cr opaque material and CrO AR material on the photomask be within specified tolerances. Thus, after a photomask is produced (as described below), the dimensions of the Cr opaque material and CrO AR material on the photomask are then measured to determine whether or not critical dimensions are within specified tolerances.
A photomask used in the production of semiconductor devices is formed from a xe2x80x9cblankxe2x80x9d or xe2x80x9cundevelopedxe2x80x9d photomask. As shown in FIG. 1, a prior art blank photomask 20 is comprised of four layers. The first layer 2 is a layer of quartz, commonly referred to as the substrate, and is typically approximately one quarter inch thick. Affixed to the quartz substrate 2 is a layer of Cr opaque material 4 which typically is approximately 900 xc3x85 to 1000 xc3x85 thick. An integral layer of CrO AR material 6 is formed on top of the layer of Cr opaque material 4, and is typically approximately 100 xc3x85 thick. A layer of photosensitive resist material 8 resides on top of the CrO AR material 6. The photosensitive resist material 8 is typically a hydrocarbon polymer, the various compositions and thicknesses of which are well known in the art.
The desired pattern of Cr opaque material to be created on the photomask may be defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. As the E-beam or laser beam is scanned across the blank photomask, the exposure system directs the E-beam or laser beam at addressable locations on the photomask as defined by the electronic data file. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble. As shown in FIG. 2, after the exposure system has scanned the desired image onto the photosensitive resist material, the soluble photosensitive resist is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 10 remains adhered to the CrO AR material 6.
As illustrated in FIG. 3, the exposed CrO AR material and the underlying Cr opaque material no longer covered by the photosensitive resist material is removed by a well known etching process, and only the portions of CrO AR material 12 and Cr opaque material 14 residing beneath the remaining photosensitive resist material 10 remain affixed to quartz substrate 2. This initial or base etching may be accomplished by either a wet-etching or dry-etching process both of which are well known in the art. In general, wet-etching process uses a liquid acid solution to erode away the exposed CrO AR material and Cr opaque material. A dry-etching process, also referred to as plasma etching, utilizes electrified gases to remove the exposed chrome oxide AR material and chrome opaque material. Dry-etching process is conducted in vacuum chamber in which gases, typically chlorine and oxygen are injected. An electrical field is created between an anode and a cathode in the vacuum chamber thereby forming a reactive gas plasma. Positive ions of the reactive gas plasma are accelerated toward the photomask which is oriented such that the surface area of the quartz substrate is perpendicular to the electrical field. The directional ion bombardment enhances the etch rate of the Cr opaque material and CrO AR material in the vertical direction but not in the horizontal direction (i.e., the etching is anisotropic or directional).
The reaction between the reactive gas plasma and the Cr opaque material and CrO AR material is a two step process. First, a reaction between the chlorine gas and exposed CrO AR material and Cr opaque material forms chrome radical species. The oxygen then reacts with the chrome radical species to create a volatile which can xe2x80x9cboil off xe2x80x9d thereby removing the exposed CrO AR material and the exposed Cr opaque material.
As shown in FIG. 4, after the etching process is completed the photosensitive resist material is stripped away by a process well known in the art. The dimensions of the Cr opaque material on the finished photomask are then measured to determine whether or not critical dimensions are within specified tolerances. Critical dimensions may be measured at a number of locations on the finished photomask, summed, and then divided by the number of measurements to obtain a numerical average of the critical dimensions. This obtained average is then compared to a specified target number (i.e., a mean to target comparison) to ensure compliance with predefined critical dimensions specifications. Additionally, it is desired that there is a small variance among the critical dimensions on the substrate. Accordingly, the measured critical dimensions typically must also conform to a specified uniformity requirement. Uniformity is typically defined as a range (maximum minus minimum) or a standard deviation of a population of measurements.
Additionally, as disclosed in co-pending application Ser. No. 09/409,454, the entire contents of which are incorporated by reference, a photomask may include a hardmask layer. As shown in FIG. 5, such a photomask is comprised, for example, of a quartz substrate 2, a patterned layer of opaque material 4, a patterned layer of antireflective material 6, and a patterned hardmask layer 18. The hardmask may be comprised of TiN, Ti, Si, Si3N4, doped and undoped SiO2, TiW, W, or spin-on-glass.
Prior art methods for measuring critical dimensions on photomasks utilize techniques based on optical microscopy or by using a scanning electron microscope (SEM). Both of these critical dimension measurement techniques have inherent disadvantages.
Techniques for measuring photomask critical dimensions based on optical microscopy can generally be performed relatively quickly and inexpensively and do not result in any significant charge being created on the photomask. However, these optical measurement techniques are not very accurate for small features and are not suitable for measuring features less than 0.6 xcexcm. Additionally, the measurement of small features can be affected by other features in close proximity to the feature being measured. Further, optical measurement techniques are subject to pitch conditions and cannot be used to accurately measure certain types of photomasks such as phase shift masks (PSMs) with various optical properties.
While critical dimensions of even small features of a photomask can be measured accurately using a SEM, the use of a SEM is relatively slow and expensive process. Further, use of a SEM can result in a charge being formed on the photomask thus reducing accuracy of the measurement
Accordingly, it is an object of the present invention to provide a fast and accurate method of measuring the critical dimensions on a photomask.
It is a further object of the present invention to provide a method for measuring the critical dimensions of a photomask using electrical test structures.
It is still a further object of the present invention to provide a method or measuring critical dimensions of a photomask with precisions less than 5 nm and for measuring critical dimensions as small as 200 nm or less.